The present invention relates to a device able to process an optical signal. More particularly the present invention relates to a device able to process an optical signal by means of an optical control system and in a manner independent of the state of polarization of this optical signal.
Following the widespread development of optical-fibre transmission systems, there is an urgent need for devices which are able to perform, at high speed, different operations on the bits which constitute the digital information of an optical signal.
The present apparatus used for the processing of optical signals are inadequate for managing the increasingly higher transmission rates which are possible in optical-fibre transmission systems. In fact, these apparatus consist of digital electronic devices which are based typically on serial processing of the information or optical devices which, however, are controlled by electrical signals. All these devices have a limited band compared to the optical band available in optical-fibre transmission systems.
Moreover, the optical signals coming from an optical transmission line have a random, and therefore unpredictable, state of polarization. In fact, during their propagation along an optical transmission means, they are subject to a random disturbance of their state of polarization (SOP). More particularly, in an optical fibre, typically a single-mode optical fibre, the SOP of an optical signal is disturbed both on account of manufacturing defects (such as, for example, a not perfectly circular geometry of the core and/or lack of homogeneity of the material) and on account of the action of external agents (such as variations in the external temperature, displacements of the fibre, vibrations and stress). These disturbances are random and unpredictable. Even a relatively short fibre (for example with a length of one metre), if subjected to stress or to variations in temperature, may disturb significantly and in a random manner the SOP of the optical signal which is propagated there.
The inventors of the present invention have therefore considered the problem of fully exploiting the optical band available in optical-fibre transmission systems using an optical device which is able to process an optical signal in optical form by means of an optical control system and in a manner independent of its state of polarization. More particularly the inventors have considered the problem of fully exploiting the optical band available in optical-fibre transmission systems using an optical device comprising a crystal element possessing properties which are electro-optical (i.e. it becomes birefringent when subject to the action of an applied electric field and its refraction indices vary with a variation in intensity in said electric field) and photo-conductive (its refraction indices, assuming a constant applied electric field, may be further varied by the action of the light which photo-generates charge carriers which screen the action of the applied electric field).
According to its first aspect, the present invention therefore relates to an optical device comprising:
a) a first input for at least one first input optical signal having a random state of polarization;
b) a first optical element capable of dividing, in space, said at least one input optical signal into a pair of optical signals which are substantially parallel and have a predetermined state of polarization perpendicular to one another;
c) a first crystal element for propagation, in free space, of at least said pair of optical signals coming from said first optical element, said first crystal element being devoid of internal separation planes, having electro-optical properties and being associated with electrodes so as to apply a voltage in a direction substantially perpendicular to the direction of propagation, along said first crystal element, of said at least one pair of optical signals;
d) an optical control element for supplying at least one optical control beam, having a predetermined power, to said first crystal element, said at least one optical control beam being superimposed on said at least one pair of optical signals and having a direction of propagation which along said first crystal element is substantially perpendicular to said direction of application of said voltage, said at least one optical control beam being capable of causing to rotate through a predetermined angle the state of polarization of said at least one pair of optical signals in said first crystal element;
e) a second optical element capable of combining said at least one pair of optical signals coming from said first crystal element in a single output optical signal;
f) a first output for said output optical signal.
During the course of the present description and the claims:
the expression xe2x80x9coptical control beam superimposed on an optical signalxe2x80x9d is used to indicate an optical control beam which is superimposed on said optical signal in the whole region crossed by it in a crystal element devoid of internal separation planes;
the expression xe2x80x9coptical control beam collinear with an optical signalxe2x80x9d is used to indicate an optical control beam which is propagated in the same direction in which this optical signal is propagated;
the expression xe2x80x9ca crystal element devoid of internal separation planesxe2x80x9d is used to distinguish a single crystal from elements obtained from the combination of two or more crystals;
the expression xe2x80x9cpropagation in free spacexe2x80x9d is used to indicate all the modes of propagation of an optical signal not guided by a suitable waveguide such as, for example, an optical fibre.
Preferably, said optical control beam is also substantially collinear with said at least one pair of substantially parallel optical signals.
Typically, said at least one pair of optical signals lies in a plane parallel to one face of said first crystal element.
Advantageously, said input consists of an optical fibre.
Typically, said first optical element comprises a right-angled reflecting prism and a polarization separator.
Similarly, said output also advantageously consists of an optical fibre. Moreover, said second optical element typically also comprises a right-angled reflecting prism and a polarization separator.
According to a first embodiment, said optical device also comprises a second output and said second optical element also comprises a second right-angled reflecting prism.
According to a second embodiment, the optical device according to said first embodiment also comprises a second input for at least one second optical signal having a random state of polarization and said first optical element also comprises a second right-angled reflecting prism.
Said first and second optical signals, which have a random state of polarization, may each be superimposed on a respective optical control beam and be propagated in two different regions of said first crystal element or, alternatively, may be associated with the said optical control beam superimposed on both of them. In this latter case, said first and second optical signals are propagated preferably in the same region of said first crystal element.
A typical example of an optical element comprising two right-angled reflecting prisms and a polarization separator is described in the patent U.S. Pat. No. 5,305,136 included herein by way of reference.
According to a variant, said first and said second optical elements may each consist of a calcite prism, for example, a calcite prism manufactured by BERNHARD HALLE with a polarization insensitivity of about xe2x88x9240 dB.
Advantageously, a suitable optical collimator is associated with each of said inputs and said outputs. Preferably, said optical collimator consists of a xe2x80x9cgrinxe2x80x9d type lens.
Preferably, said first crystal element consists of a single crystal of cadmium telluride doped with indium (CdTe:In).
A typical example of said first crystal element is described in the European patent application No. 97201874.1, filed in the name of the same Applicant, which is included herein by way of reference.
Preferably, said voltage applied to the electrodes of said first crystal element is selected so as to rotate through an angle of about 90xc2x0 the state of polarization of said at least one pair of substantially parallel optical signals.
Moreover, the power of said optical control beam is preferably selected so as to cancel out the effect of said voltage, causing said at least one pair of substantially parallel optical signals to return to their initial state of polarization.
Typically, said optical control element comprises an optical source and a collimation element. According to one embodiment, said optical control element also comprises a first and a second dichroic mirror. Preferably said first dichroic mirror is arranged upstream of said first crystal element and said second dichroic mirror downstream thereof. Moreover, preferably said dichroic mirrors are transparent to the wavelength of said at least one pair of optical signals and therefore reflect the wavelength of said optical control beam.
Preferably, the wavelength of said optical signal with a random state of polarization is between 1000 and 1650 nm and, more preferably, between 1300 and 1600 nm.
Advantageously, said dichroic mirrors are substantially inclined at 45xc2x0 with respect to the direction of propagation of said at least one pair of optical signals and said optical control beam. Moreover, said optical control beam has a direction substantially perpendicular to the direction of propagation of said at least one pair of optical signals both upstream of said first crystal element, prior to striking said first dichroic mirror, and downstream of said first crystal element, after being reflected by said second dichroic mirror.
Preferably, said element for collimation of said optical control element is capable of directing in free space said optical control beam so that it strikes said first dichroic mirror at about 45xc2x0 and so that, after being reflected by it, it is substantially collinear with and superimposed on said at least one pair of optical signals.
Even faster response times may be obtained when the optical device according to the invention also comprises a second crystal element positioned between said first crystal element and said second optical element and when said optical control element also supplies a second optical control beam to said second crystal element.
As regards the structural and functional characteristics of said second crystal element, reference is made to that already stated above in connection with said first crystal element.
According to a third embodiment, in the optical device according to the invention:
first optical elements are capable of dividing two first optical signals, which have a random state of polarization, into two respective first pairs of substantially parallel optical signals, which have a predetermined state of polarization perpendicular to one another, and of causing propagation of said first pairs of optical signals in a first region of said first crystal element, and
second optical elements are capable of dividing two second optical signals, which have a random state of polarization, into two respective second pairs of substantially parallel optical signals which have a predetermined state of polarization perpendicular to one another, and of causing propagation of said second pairs of optical signals in a second region of said first crystal element.
Preferably, the optical control element also supplies a second optical control beam which illuminates said second region of said first crystal element, while said first optical control beam illuminates said first region of said first crystal element. Moreover, said second region is different from said first region.
Typically, said first elements and said second elements comprise, in addition to the first and second optical element also a third and a fourth optical element and four optical circulators each with three ports.
Advantageously, each optical circulator has a port associated with one of the two inputs or with one of the two outputs of the optical device according to the invention and two ports associated with a respective optical collimator so as to cause propagation of said first pairs of optical signals in said first region of said first crystal element and said second pairs of optical signals in said second region of said first crystal element.
As regards the structural and functional characteristics of said third and fourth optical elements, said collimators and said second optical control beam, reference is made to that described further above.
In addition to the advantage of being insensitive to the state of polarization of the input optical signals and of being optically controlled, the optical device according to the invention has the advantage of processing at the same time several optical signals which are polarized by means of one or more optical control beams in a single crystal element devoid of internal separation planes.
This has the advantage of allowing the manufacture of a device which is much more compact and has a simpler architecture compared to the use of as many single crystals as there are polarized optical signals to be processed, each provided with independent electrodes.
Moreover, the device according to the present invention ensures uniformity of performance for each signal, avoiding having to select several crystals with characteristics which are identical to one another.
Furthermore, since the device according to the invention allows the processing of optical signals in a wide range of wavelengths (1000-1700 nm), it is suitable for use in a wavelength multiplexing telecommunications system (Wavelength Division Multiplexing, WDM).
For example, in a preferred embodiment which uses a CdTe:In single crystal, the optical device according to the present invention allows the processing of optical signals with a wavelength greater than 1200 nm and, therefore, also with wavelengths typical of optical-fibre transmission systems in a second and third window (approx. 1300-1550 nm).
Furthermore, the device according to the invention is characterized by a response time which is much faster than that of a similar electrically controlled device. For example, the device according to the invention has a response time of the order of microseconds and, in a preferred embodiment, of the order of nanoseconds, while the typical response times of an electrically controlled device are of the order of milliseconds.
Finally, another characteristic feature of the device according to the invention is that, owing to the symmetrical structure of the paths followed by the two optical signals polarized in mutually perpendicular states of polarization, at the output from the crystal element, when they are recombined by the optical element, the two optical signals are not delayed temporally with respect to one another.